1. Field of the Invention
The present invention relates to a transmission electron microscope (TEM).
2. Description of the Prior Art
FIG. 4 schematically shows a high-spatial resolution TEM. This microscope has an electron gun 1, an accelerating portion 2 for accelerating electrons, an aperture 3, condenser lenses 4, objective lenses 5 and a specimen stage 6. Projector lenses, a fluorescent screen, and other components located downstream of the specimen stage 6 are omitted in FIG. 4. The side of the electron gun as viewed in the direction of the optical axis is referred to as the "upstream" side herein. The opposite side is referred to as the "downstream" side herein. Accordingly, the condenser lenses 4 are positioned upstream of the objective lenses 5 and downstream of the accelerating portion 2.
The electron beam emitted by the electron gun 1 is accelerated with a given accelerating voltage by the accelerating portion 2. In the high-spatial resolution TEM, it is usual to set the accelerating voltage to more than 200 kV. Therefore, the electron beam is accelerated with a given accelerating voltage of more than 200 kV. The accelerating portion 2 is usually composed of plural accelerating tubes to avoid internal electric discharge. Each individual accelerating tube is omitted in FIG. 4; rather these tubes are collectively shown as one block.
FIG. 5 shows the structure of the accelerating portion 2 using an LaB.sub.6 gun that is one kind of thermionic electron gun. In this structure, the gun is composed of a first anode and the following five accelerating electrodes, or five accelerating tubes. A field emission gun (FEG) is often used as the electron gun 1.
The electron beam accelerated with the given accelerating voltage by the accelerating portion 2 passes through the aperture 3 and is converged by the condenser lenses 4. The beam then reaches the specimen stage 6 through the objective lenses 5. Usually, the condenser lenses 4 are composed of plural stages of lenses. However, the lenses are collectively shown as one block in FIG. 4. Similarly, the objective lenses 5 are shown as one block.
The electron guns of these structures have been improved. Also, the structures of the various lenses typified by the objective lenses 5 have been studied to improve the spherical aberrations and the chromatic aberrations. As a result, the spatial resolution has been improved. Since the present invention does not lie in improvements in electron guns, they are not discussed in detail herein. It can be considered that the spherical aberrations of the objective lenses 5 have been reduced to an infinitesimal level. However, we have found that there remains room for improvements in chromatic aberrations.